The present invention relates generally to a loss compensation circuits in telecommunication transmission facilities and, more particular, to a trans-hybrid loss compensation circuit which varies between various modes of operation depending on whether an associated telephone is on hook or off hook.
The Bell telephone system in the United States, for example, has widely utilized digital "D" multiplexing code modulation systems. A "D" channel bank, for example, commonly provides multiple DS-1 signals that are carried on a T-1 transmission system. One pair of cables is provided for each direction of transmission.
The facilities generally include the central office switch, which provides an analog signal to a central office channel unit. The central office channel unit converts the two wire analog signal to a digital data signal transmitted on two pairs of cables. The digital data signals are transmitted to a remote transmission channel unit. The digital data signal is then reconverted to an analog signal and provided, in analog form, on two a wire conductor, or Tip-Ring pair, to the customer's premises.
Signals which are sent via digital carrier transmission lines from the central office reach a remote terminal unit before reaching the customer premises. The remote terminal then converts the digital signal to an analog signal, which may then be an intelligible signal for the telephone. Thus, in the remote terminal, the digital data signals, sent over the T-1 lines, are converted to analog telephone signals and then supplied to a customer loop telephone line. The analog telephone signals may then be received by the customer premises, which may include telephones and, in some cases, PBX units. Conversely, the remote terminals receive analog telephone signals from the telephones and PBX units and convert them to digital data signals, which then can be transmitted over the T-1 transmission system.
The customer's premises may include a telephone and a caller identification box which deciphers information sent by the central office regarding the telephone number of the party originating the call. Such information is normally found in the signal provided to the customer's telephone after the first ring of the telephone.
Many transmission lines include a hybrid circuit that interfaces a two-wire transmit transmission line and a two-wire receive transmission line (a four-wire circuit) to a the Tip-Ring pair. Part of the signal on the receive transmission line may be reflected back to the transmit transmission line. This, in turn, may be reflected back along the receive transmission line, resulting in substantial "noise" in the transmission line system. A common practice is to invert the received signal and add it to any transmit signal received from the telephone lines (to effectively cancel out "reflected" signals). The level of the transmitted signal is compared to the receive signal and represented in dB: dB=20 Log (Transit Voltage/Receive Voltage). Such measures are not always effective in eliminating reflected signals.
Some customer premises equipment, such as a caller identification boxes, are sensitive to "reflected noise." For example, caller identification boxes generally receive information between the first and second rings on a telephone. Reflected noise in the transmission line system may interfere with a caller identification box's ability to correctly interpret information transmitted over the telephone lines between rings.